The present invention relates to an electric power steering apparatus in which a motor is used as a source of a steering assist force.
Steering of an automobile is carried out by transmitting rotating operation of a steering wheel disposed within a vehicle room to a steering mechanism disposed on an outside of the vehicle room for steering wheels (front wheels, in general) for steering.
FIG. 1 is a sectional view of an electric power steering apparatus of prior art and FIG. 2 is a sectional view of a reduction mechanism and a motor portion.
As shown in FIG. 1, the electric power steering apparatus for the automobile has a first steering shaft 102 connected to a steering wheel 101, a torque sensor 105 for detecting steering torque based on a relative displacement between the first steering shaft 102 and a second steering shaft 104 connected to the first steering shaft 102 through a torsion bar 103 in a rotating direction, and the reduction mechanism 109 including a worm 107 and a worm wheel 108 for increasing a rotating force of a steering assisting motor 106 driven based on a detection result of the torque sensor 105 and for transmitting the rotating force to the second steering shaft 104. By means of the above constitution the electric power steering apparatus assists operation of the steering mechanism according to rotation of the steering wheel 101 by rotation of the motor 106 to reduce labor and a load imposed on a driver for steering.
Opposite end sides of the worm shaft 110 provided with the worm 107 and the second steering shaft 104 provided with the worm wheel 108 in shaft length directions are respectively supported by bearings 111, 112, and 113, 114 and movements of the shafts in diameter directions and the shaft length directions are prevented.
If the worm 107 and the worm wheel 108 are used as described above, a distance between rotation centers of the worm shaft 110 and the worm wheel 108 is preset in assembly. In this presetting, dimensional errors of the worm 107, the worm shaft 110, the bearings 111 and 112 for supporting the worm shaft 110, the worm wheel 108, the second steering shaft 104 for supporting the worm wheel 108, and the like cause variations in the distance between the rotation centers, backlash occurs after assembly, and noise due to the backlash is heard in the room of the automobile.
A meshing state changes with time because cogs of the worm 107 and the worm wheel 108 wear as a result of steering or because the worm wheel 108 made of synthetic resin contracts due to low temperatures during wintertime and the like and occurrence of the backlash is inevitable.
In order to reduce variations in the distance between the rotation centers in assembly, it is necessary to process parts such as the worm shaft 110, the bearings 111 to 114, the worm wheel 108, and the second steering shaft 104 with small dimensional errors, measure dimensions of all the parts, select the respective parts based on the measured dimensions, and assemble the parts. Therefore, costs of processing and assembly and cost of the entire electric power steering apparatus increase. Moreover, because the movements of the assembled worm shaft 110 and worm wheel 108 in directions intersecting the rotation centers are prevented, the distance between the rotation centers cannot be adjusted and the backlash cannot be adjusted when the meshing state has changed with time.